The present invention relates to the field of semiconductor device structures, especially transistor, diode and pixel structures, more particularly to a pixel structure, that does not show a degradation when irradiated with ionizing radiation. The present invention also relates to a method for manufacturing such a semiconductor device structure.
Ionizing radiation is known to cause damage to transistors comprised on a semiconductor surface e.g. as in a pixel arrangement of a display device or camera, which typically comprises a photodiode and a few transistors for resetting the photodiode and to read out the signal. The pixel structures of the state of the art usually comprise several transistors that comprise a source and a drain region, separated by a gate-oxide under a polysilicon gate electrode and surrounded by field oxide. The polysilicon gate electrode usually overlaps part of the field oxide, creating a parasitic MOSFET. Under normal circumstances, this does not present a problem since the threshold voltage of this MOSFET lies much higher than the threshold voltage of the actual transistor.
Ionizing radiation will typically lead to a shift of the threshold voltage, and this effect will be, depending on the voltage of the field oxide, more pronounced in the MOSFET than in the actual transistor, possibly creating a leak path. This will result in a transistor that cannot be switched off any more. The extent of these effects also depends on the thickness and quality of the oxides and on the magnitude of the electric field.
Many semiconductor devices suffer from radiation sensitivity. This is particularly true for electronic devices used in space, where they may be subject to ionizing radiation such as cosmic rays. Ionizing radiation may include high energy electromagnetic radiation like xcex3 rays or high energy particles. Pixels and other semiconductor devices must be sensitive to electromagnetic radiation of one type (e.g. visible light, UV light, infra-red light) or must be visible if they are part of a display device which is to be viewed. Thus, protective shielding, such as a metal layer, cannot be placed around such devices. This means that resistance to ionizing radiation must be provided by some other means. Further, where large arrays of pixels are fabricated using VLSI techniques it is difficult to provide protective layers at the pixel level, e.g. by depositing thick metal layers locally to a pixel during semiconductor processing.
An aim of the present invention is to provide a radiation resistant semiconductor device structure.
A further aim of the present invention is to provide a method for manufacturing such a radiation resistant semiconductor device structure.
A first aspect of the present invention concerns a semiconductor transistor device structure for reducing radiation induced current flow caused by incident ionizing radiation, comprising:
a semiconductor substrate;
two or more regions of a first conductivity type in the substrate; and
a doped guard ring of a second conductivity type surrounding the two or more regions of a first conductivity type for obstructing radiation induced parasitic current flow between the two or more regions of the first conductivity type. The guard ring is a ring interrupted by an active area of the substrate controlled by a gate electrode.
The present invention may also provide a pixel sensitive to a radiation of a first type with reduced radiation induced current flow caused by incident ionizing radiation of a second type, comprising:
a semiconductor substrate;
two or more regions of a first conductivity type in the substrate; and
a doped guard ring of a second conductivity type surrounding the two or more regions of a first conductivity type for obstructing radiation induced parasitic current flow between the two or more regions of the first conductivity type. The guard ring is a ring interrupted by an active area of the substrate controlled by a gate electrode.
The present invention may also provide a semiconductor transistor structure including a semiconductor substrate, two or more regions of a first conductivity type in the substrate, and a doped guard ring of a second conductivity type surrounding the two or more regions of a first conductivity type for obstructing radiation induced parasitic current flow between the two or more regions of the first conductivity type, the guard ring being a ring interrupted by an active area of the substrate controlled by a gate electrode. The region of a first conductivity type comprises a source part and a drain part separated by a junction, and the gate electrode covers at least part of said junction, wherein said second region surrounds said first region but is not in direct contact with said first region.
The present invention may also provide a photodiode device comprising:
a semiconductor substrate,
a first region of a first conductivity type in the substrate,
a gate electrode surrounding said first region, and
a doped guard ring of a second conductivity type surrounding said gate electrode, wherein the guard ring is a ring interrupted by an active area of the substrate controlled by the gate electrode.
The present invention will now be described with reference to the following drawings.